Ammonium nitrate propellant compositions containing nitrilotriacetate combustion catalyst



United States Patent Ofiice p amass Patented Dec. 3, 1963 This invention relates to ammonium nitrate-type compositions and particularly a catalyst for promoting the combustion of ammonium nitrate containing compositions.

In gas generation and rocketry usages it is necessary that the gas affording composition develop gas at a uniform rate; in the art this is spoken of as burning at a uniform rate. In ammonium nitrate compositions which consist essentially of ammonium nitrate particles and an oxidiziable organic material which permits the shaping of the composition into a definite configuration or grain, it is necessary to promote the combustion of the composition by the use of a combustion catalyst. Commonly used catalysts are the inorganic chromium compounds, particularly ammonium dichromate. In the military field, the Prussian blues have attained eminence. These and other burning catalysts comprising heavy metals have the drawback of forming solid oxide products in the combustion gases which are believed to be the cause of nozzle erosion. Nozzle erosion results in erratic change in gas pressure within the gas generator or rocket motor.

'It has been found that a suitable ammonium nitratetype composition for gas generator and rockctry usage is obtained with ammonium nitrate as the predominant component, between about and 40 weight percent of oxidizable organic material and, as the combustion catalyst, between about /2 and 10 weight percent of an alkali metal salt, an alkaline earth metal salt, or mixtures thereof, of nitrilotriacetic acid (N(CH COOH) The salts may he mono-, di-, or tri-alkali metal or alkaline earth metal salts; the alkali metal salts are preferred. These salts are nontoxic, noncorrosive, easy to handle, and are effective as burning catalysts at low concentrations in an ammonium nitrate-type composition. In addition, they improve the storage stability characteristics of the composition, and cause relatively little nozzle erosion.

Sufiicient catalyst must be introduced into the composition to promote the burning of the composition. The amount of catalyst used is also influenced by the rate of burning desired. The more catalyst present, the faster the combustion of the composition. It is to be understood that the burning rate is also affected by the particular oxidizable organic material present. In general, the composition will contain between about 0.5 and 10 weight percent of the catalyst. (Hereinafter all percentages are to be understood as weight percent.) With the thermoplastic matrix formers or binders obtained from cellulose esters and oxygenated hydrocarbon plasticizers therefor,

between about 1 and 6% of catalyst produces satisfactory burning rates for typical military gas generation and rocketry usages; more usually 24% is present.

The improved composition of the invention contains ammonium nitrate as the major component. The ammonium nitrate may be ordinary commercial ammonium nitrate such as is used for fertilizers. This commercial grade material contains a small amount of impurities and the particles are usually coated with moisture-resisting material such as parafiin wax. Military grade ammonium nitrate which is almost chemically pure is particularly suitable. The ammonium nitrate is preferably in a finely divided particulate form which may be either produced "by prilling or by grinding. The ammonium nitrate is the major component of the gas-generator composition and usually the composition will contain between about 55% and ammonium nitrate.

In order to permit the shaping of the ammonium nitrate composition into definite configurations, a matrix former or binder material is present. When ammonium nitrate decomposes, free-oxygen is formed. Advantage of the existence of this free-oxygen is taken, and oxidizable organic materials are used as the binders. These oxidiz-able organic materials may contain only carbon and hydrogen, for example, high molecular weight hydrocarbons such as 'asphalts or residuums, and rubbers, either natural or synthetic. Or, the oxidizable organic material may contain other elements in addition to carbon and hydrogen, for example, as in Thiokol rubber and neoprene. The stoichiometry of the composition is improved, with respect to smoke production, by the use of oxygenated organic materials as the binders. The binder or matrix former may be a single compound such as a rubber or asphalt or it may be a mixture of compounds. The mixtures are particularly suitable when special characteristics are to be imparted to the grain which cannot be obtained by the use of a single compound.

The multi-component binder or matrix former commonly consists of a polymeric base material and a plasticizer therefor. Particularly suitable polymeric base materials are cellulose esters of alkanoic acids containing from 2 to 4 carbon atoms such as cellulose acetate, cellulose acetate butyrate and cellulose propionate; the polyvinyl resins such as polyvinylchloride and polyvinyl acetate are also good bases; styrene acrylonitrile is an example of a copolymer which forms a good base material. In general, the binder contains between about 15% and 45% of the particular polymeric base material.

The plasticizer component of the binder is broadly defined as an oxygenated hydrocarbon. The hydrocarbon base may be aliphatic or aromatic or may contain both forms. The oxygen may be present in the plasticizer in ether linkage and/ or hydroxyl group and/ or carboxyl groups; also the oxygen may be present in inorganic substituents, particularly nitro groups. In general, any plasticizer which is adopted to plasticize the particular polymer may be used in the invention. Exemplary classes of plasticizers which are suitable are set out below.

It is to be understood that these classes are illustrative only and do not limit the types of oxygenated hydrocarbons which may be used to pla-sticize the polymer.

Di-lower alkyl-phthalates, e.g., dimethyl phthalate, dibutyl phthalate, dioctyl phthalate and dimethyl nitrophthalate.

Nitrobenzenes, e.g., nitrobenzene, dinit-robenzcne, nitrotoluene, dinitrotoluene, nitroxylene, and nitrodiphenyl.

Nitrodiphenyl ethers, e.g., nitrodiphenyl ether and 2,4-

dinitrodiphenyl ether.

Tri-lower alkyl-citrates, e.g., triethyl citrate, tributyi citrate and triarnyl citrate.

Acyl tri-lower alkyl-citrates where the acyl group contains 2-4 carbon atoms, e.g., acetyl triethyl citrate and acetyl tributyl citrate.

Glycerol-lower alkanoates, e.g., monoacetin, triacetin,

glycerol tripropionate and glycerol tribu-tyrate.

Lower -alkylene-glycol-lower al'kan-oates wherein the glycol Lower alkylene-glyccls wherein the molecular weight is below about 200, e.g., diethylene glycol, polyethylene glycol (200), and tetrapropylene glycol.

Lower alkylene-glycol oxolates, e.g., diethylene glycoloxolate and polyethylene glycol (200) oxolate.

Lower alkylene-glycol maleates, e.g., ethylene glycol maleate and bis-(diethylene glycol monoethyl ether) maleate.

Lower algylene-glycol diglycolates, e.g., ethylene glycol diglycolate and diethylene glycol diglycolate.

Miscellaneous diglycollates, e.g., dibutyl diglycollate, dimethylalkyl diglycollate and methylcarbitol diglycollate.

Lower alkyl-phthalyl-lower alkyl-glycollate, e.g., methyl phthalyl ethyl glycollate, ethyl phthalyl ethyl glycolate and butyl phthalyl butyl glycollate.

Di-lower a kyloxy-te-traglycol, e.g., dime-thoxy tetra glycol and dibutoxy tetra glycol.

Nitrophenylether of lower alkylene glycols, e.g., dinitrophenyl ether of triethylene glycol and nitrophenyl ether of polypropylene glycol.

Nitrophenoxy alkanols wherein the alkanol portion is derived from a glycol having a molecular weight of not more than about 200. These may be pure compounds or admixed with major component bis(nitrophenoxy) lkane.

A single plasticizer may be used or more usually two or more plasticizers may be used in conjunction. The particular requirements with respect to use will determine not only the polymer but also the particular plasticizer or combination of plasticizers which are used.

In addition to the basic components, i.e., ammonium nitrate binder and catalyst, the gas-generator propellant composition may contain other materials. For example, materials may be present to improve low temperature ignitability, for instance, oximes may be present or asphalt may be present. Surfactants may be present in order to improve the coating of the nitrate with the binder and to improve the shape characteristics of the composition. Various burning rate promoters, such as finely divided carbon, which are not catalysts per se, may also be present.

The aromatic hydrocarbon amines are known to be gas evolution stabilization additives. Examples of these aromatic amines are toluene diamine, diphe-nyl amine, naphthalene diamine, and toluene triamine. In general, the aromatic hydrocarbon amines are used in amounts between about 0.5 and percent. While these aromatic hydrocarbon amines are efiective, for severe duties they are frequently not sufiiciently elfective alone. It has been found that extremely good stabilization is obtained when N-phenylmorpholine additive is used with an arcmatic hydrocarbon amine. Because of the plasticizing power of the N-phenyl-morpholine it is generally desirable to use the aromatic hydrocarbon amines as the primary stabilizing additive and the N-phenylmorpholine in an amount needed to obtain the specific stability. In general, when aromatic hydrocarbon amines are present between about 0.1% and 1% of N-phenylmorpholine will be used.

Broadly the composition will contain between about 20 and 35 weight percent of binder when the polymeric base material is a cellulose ester of an alkanoic acid containing 2 to 4 carbon atoms and an oxygenated hydrocarbon plasticizer therefor. A particularly useful composition consists of cellulose acetate, about 6-12%; acetyltriethylcitrate, about 612%; a mixture having two to four parts of dinitrophenoxyethanol to one part of bis(dinitrophenoxy)ethane, about 642%; carbon, about 24%; toluene diamine, about /9.%; N-phenylmorpholine, about /2 and catalyst, about 24%.

4 TESTS Two compositions were tested for burning rate, storage stability, and other characteristics required by military specifications. Composition A contained no burning catalyst; Composition B contained 3.0 weight percent potassium nitrilotriacetate as the catalyst.

Each composition was prepared by mixing together for one hour in a laboratory mixer a 300-gram batch having ingredients in the proportions indicated in the table set forth below. The mixing temperature was about C. Lacquer grade commercial cellulose acetate analyzing about 55 percent of acetic acid equivalent was the polymer base. Two plasticizers were used. One plasticizer contained about three parts of dinitrophenoxy ethanol and one part of bis(dinitrophenoxy)ethane, obtained by the reaction of dinitrochlorobenzene and ethylene glycol in the presence of aqueous sodium hydroxide solution. The other plasticizer was acetyl triethyl citrate. Toluene diamine and N-phenylmorpholine were used as stabilizers.

The potassium nitrilotriacetate used as the catalyst in Composition B was prepared by adding potassium hydroxide to nitrilotriacetic acid, in a 3:1 molar ratio, diluting the resulting mixture with water, and evaporating to dryness.

After mixing the resulting pasty mass was compression molded into a slab approximately one-half inch in thickness. The slab was subsequently sawed into strips for the burning rate test and broken into smaller pieces for the storage stability test.

The burning rate tests were conducted in a Crawford bomb pressured at 1000 p.s.i.g. and 25 C.

The high temperature storage stability of the compositions was determined in a laboratory test as follows. A small sample, about three grams, of the composition was placed in a vessel connected by tubing to a mercury manometer system which was so arranged that differential readings of the manometer were translatable into volume changes in the system. Since volume change of the composition sample itself can be disregarded, the volume change in the system corresponds to the amount of gaseous decomposition products from the sample. The vessel was inserted into a metal block provided with electrical heating elements and controls which permit the block to be maintained at a temperature of C. A period of 15 minutes was allowed for the sample to come to the temperature of 150 C., at which time the manometer was zeroed. While the sample was maintained at 150 C., the time (induction period) was measured between the zeroing of the manometer and the time when the gas evolution rate reached one cc. per gram per hour. Also the time was measured between the zeroing of the manometer and the time at which a gas evolution rate of five cc.s per gram per hour was reached.

The following table summarizes the ingredients of each composition tested and the results of the burning rate and high temperature stability tests conducted theeron:

Table 1 Composition A 13 Ingredients, weight percent:

Ammonium nitrate G2. 9 (51.0 Cellulose acetate 10 5 1 o Dinitrophcnoxy ethanol and bis(dinitrophenoxy) ethane 10. 5 10. 2 Acetyltriethyl citrat 12.0 11. (1 Carbon black 3.1 3.0 Toluene iliamine, 0. 5 0. 5 Nahenyl morph oli 0. 5 (J. 5 Potassium nitrilotriacetate 0 3. 0

Totals 100.0 100 0 Test Results:

Burning Rate, inches/sec 0. 055 0. 033 Pressure Exponent a. 0. 60 0. 36 lnluction Time, hours O. 5 5. 3 Time to gas evolution rate of 5 ecJg. 0. T5 5. 9

Thus having described the invention, what is claimed is:

1. A composition consisting essentially of between about 0.5 and 10 weight percent of a combustion catalyst selected from the class consisting of alkali metal salts, alkaline earth metal salts, and mixtures thereof, of nitrilotriacetic acid, ammonium nitrate as the predominant component and between about 10 and 40 weight percent of oxidizable organic binder material wherein said binder material consists of a polymeric base selected from the class consisting of cellulose esters of alkanoic acids containing from 2 to 4 carbon atoms, polyvinyl chloride, polyvinyl acetate, and styrene-acrylonitrile, and an oxygenated hydrocarbon adapted to plasticize said polymer.

2. A composition consisting essentially of ammonium nitrate as the predominant component, between about 1 and 6 weight percent of an alkali metal salt of nitrilotriacetic acid, between about 20 and 36 weight percent of a binder consisting of a cellulose ester of an alkanoic acid containing 2 to 4 carbon atoms and an oxygenated hydrocarbon adapted to plasticize said polymer.

3. A composition consisting essentially of (a) ammonium nitrate, (b) cellulose acetate, about 6-l2%, (c) acetyl .triethyl citrate, about 6-12%, (d) about 6-12'% of an about 2:1 mixture of dinitrophenoxyethanol and bis(dinitrophenoxy)ethane, (e) carbon, about 2-4%, (f) toluene diamine, about /z%, (g) N-phenylmorpholine, about /2%, and (h) a potassium salt of nitrilotriacetic acid, about 24%.

References Cited in the file of this patent UNITED STATES PATENTS 2,942,963 Burgwald June- 28, 1960 

1. A COMPOSITION CONSISTING ESSENTIALLY OF BETWEEN ABOUT 0.5 AND 10 WEIGHT PERCENT OF A COMBUSTION CATALYST SELECTED FROM THE CLASS CONSISTING OF ALKALI METAL SALTS, ALKALINE EARTH METAL SALTS, AND MIXTURES THEREOF, OF NITRILOTRIACETIC ACID, AMMONIUM NITRATE AS THE PREDOMINANT COMPONENT AND BETWEEN ABOUT 10 AND 40 WEIGHT PERCENT OF OXIDIZABLE ORGANIC BINDER MATERIAL WHEEIN SAID BINDER MATERIAL CONSISTS OF A POLYMERIC BASE SELECTED FROM THE CLASS CONSISTING OF CELLULOSE ESTERS OF ALKANOIC ACIDS CONTAINING FROM 2 TO 4 CARBON ATOMS, POLYVINLY, CHLORIDE, POLYVINYL ACETATE, AND STYRENE-ACRYLONITRILE, AND AN OXYGENATED HYDROCARBON ADAPTED TO PLASTICIZE SAID POLYMER. 